Preparation of electric blasting cap mixture containing amorphous boron and lead oxide



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INVENTORS. GUY BYAR YOU/VG Figl 5l; E' VE N JAMES LUBINSK/ TTORNEY United States Patent Otice 3,317,36@ Patented May 2, 1967 1 Claim. (Cl. 149-2) This application is a division of copending application Serial No. 442,714, iled Mar. 25, 1965.

The invention relates generally to electric detonator systems. More particularly, it relates to (a) a novel ignition charge for electric blasting caps, (b) a method for preparing said ignition charge and (c) yan electric blasting cap of high resistance to stray electric currents and electrostatic discharges.

As is well known, electric blasting caps generally contain a base charge of a detonating explosive, such as PETN (pentaerythritol tetranitrate), a heat sensitive initiation charge, such as DDNP (diazodinitrophenol), super-imposed on the base charge, and a readily ignitable ignition charge capable of firing the initiation charge by the heat generated. Delay electric blasting caps may also contain a charge of slow-burning mixture or a length of delay Ifuse positioned between the ignition charge and the initiation charge to provide a specified time between the application of the liring current and explosion of the base charge.

Electric blasting caps have been used with advantage for many years. However, their use has involved certain hazards not encountered when fuse caps were used. A particularly dangerous hazard is that of premature detonation caused by stray electric currents or electrostatic discharges. In `attempting to offset these hazards, various Atypes of ignition charges and/or constructible designs of blasting `caps have been proposed. To date, however, no

economically feasible blasting cap has been proposed which provides complete protection against these hazards.

The present invention is concerned with the provision of a blasting cap with markedly superior resistance to these hazards. I

Accordingly, it is the primary object of the invention to high resistance to stray electric currents and electrostatic discharges. It is afurther object to` provide, as the ignition charge for said cap, a composition having unusual and advantageous properties. A still further object is the pro- It has been found that these three types of mixtures exhibit unusually high levels of voltage breakdown; hence, when used in electric blasting caps designed with a short arc path between the leading wires and the shell, but outside the locus of any sensitive component, they provide unusually high resistance to stray electric currents or static charges normally encountered in the use of the caps. Also, the three types of mixtures burn in a non-violent manner and without formation of combustion gases or development of excessive heat whereby they are particularly suitable for use in blasting caps without the need of a gas Vexpansion chamber.

The three types of mixtures .also exhibit ready and consistent ignitability by a hot bridgewire of given composition and dimensions which has been heated by the flow of a limited current of specified magnitude and duration. This property of consistent bridgewire sensitivity is most unusual in view of the wide variation in compositions which the three types of mixtures represent. Usually the bridgewire sensitivity of such fuel-oxidizer mixtures changes significantly with only minor variations in the proportions of the components. It is essential that consistent bridgewire sensitivity be maintained in the manufacture of electric blasting caps in order for the caps to be fired in series without failures, particularly when large numbers of caps are use'd in a blast. Also, hazardous misfires can occur in a series circuit when the bridgewire sensitivity of the caps is not uniform.

As has `been indicated, .an important feature of the present invention is the method of preparing the ignition compositions in granular, free-flowing form, without the employment of a graining agent. Essentially, this novel process involves the following steps. The required proportions of the finely divided amorphous boron and the PbO, or Pb304, are combined by a wet mixing process to form an intimate uniform mixture. This may be done in a mill jar with suitable balls and a liquid medium, such as acetone or Water, or a mixture of acetone and water. After drying, the resultant cake is crushed and forced through a screen of 20-60 mesh size. The screened material is then placed in a dielectric container such as a jar made of glass, plastic or other non-conductive material,

and electrostatically grained to substantially spherical form by tumbling for a period ofat least about 1 hour.

` provide a practical blasting cap which exhibits unusually vision of a method for forming the ignition charge in a l Vfree-flowing granular form without the use of a :graining agent. Other and further objects will be apparent from the following detailed description of the invention.

In accordance with one aspect of the invention, a novel ignition charge for a static and stray current resistant blasting cap is provided by forming an intimate, uniform I mixture of amorphous boron and PbO or Pb304 in certain defined proportions and in such a manner as to provide essentially spherical, free-flowing Aaggregates thereof. The proportions of boron and PbO or Pb304, of which these mixtures are comprised, are as follows:

No'material graining agent, or binder, is required.

The bulk density of the grained ignition mixture is controlled within selected limits, i.e., from- 1.2 to 3.0 g./cc. by varying the initial particle size, the diameter of the jar, the 'speed of rotation land the duration of the rotation.

This control of vbulk density is necessary to permit the use -of volumetric chargers during manufacturing operations. f Typical preparational procedures are illustrated in the following specific examples in which all parts are by Weight.

Example I Ten grams of 90-97% purity amorphous boronland 490 grams of -5 micron PbO of 99+% purity are placed Y in a ceramic mill jar of 1/2 gallon capacity containing 1 kg. of 1i-inch diameter and 1 kg, 1/z-inch diameter stainless steel balls, and 0.7 kg. water and mixed for 16 hours. Following mixing, the Wet mixture is poured through a 1a-inch mesh screen Vto remove 4the balls. The wet mixture is caught in a 2-inch x lO-inch x l2-inch stainless steel .pan and dried to a moisture content of not more than 0.1% in a suitable forced-draft electric oven operating at 212 to 225 F. over a period of approximately four hours. After drying, the resultant cake is crushed with a non-sparking spatula (soft -rubber preferred) and forced through a 20-mesh screen using the same spatula. The 500 g. of screened particles are t-hen placed in aglass jar l0 inches in length and 6 inches in rdiameter and f for -heat motors and the like.

3 electrostatically grained by rotating the jar on its lengthwise axis at 65 r.p.m. for 2.25 hours. The grained material has a bulk density of 210.2 g./cc.

Examples 43); and, 2.5/ boron/97.5% PbO` (Example 4).

Example 5 A more rapid burning composition than that described in Example l, 4but with the same desirable properties, results when parts, by weight, of 90-97% purity amorphous boron and 90 parts, by weight, of -5 micron PbO of 99\-% purity is prepared according to the procedure of IExample l. This mixture is especially suitable for use :as the ignition mixture in delay caps designed to function in less than 300 milliseconds. It releases an average Aof 217 calories per gram as compared to 133 calories per gram for the 2% boron/98% PbO mixture of Example 1. The bulk density is 1.5 $0.2 g./cc. when grained as described.

Examples 6-10 Additional examples of type 2 (supra) mixtures of the invention, prepared according to the procedures and specications of Example 1, except for the proportions of boron and PbO used, are the following: 8% boron/92% PbO (Example 6); 15% boron/85% PbO (Example 7); 20% boron/'80% PbO (Example 8); 25% boron/75% PbO (Example 9); and, 30% boron/70% PbO` (Example 1-0).

Example 11 A composition composed of 25 parts by weight of 90- 97% purity amorphous boron `and 75 parts by Weight of -5 micron PbBO.,z of not less than 90% purity was prepared by the procedure of Example 1. This composition burns at about the same rate as the 10% boron/ 90% PbO but releases an average of 248 calories per gram. Hence, this mixture is most suitable as the ignition mixture for very rapid delays or the ignition-energy charge Bulk density is approximately 1.2 g/cc.

Examples 12-15 Additional examples of type 3 (supra) mixtu-res of the invention, prepared as in Example 11, except for the proportions of boron and Pb304 used, are the following: 8% boron/92% Pb304 (Example 12); 15% boron/85% Pb304 (Example 13); 20% boron/180% P12304 (Example 14); and, 30% lboron/70% Pb304 (Example 15).

As opposed to the foregoing examples of ignition mixtures of this invention, mixtures of boron and PbO, or P13304, in proportions a-t or near stoichiometric levels, when initiated by a hot bridgewire, spark or flame, react more violently than the heat liberated would indicate. Examples of these are the following.

Example 16 A stoichiometric mixture of boron and lead monoxide was prepared by the procedure of Example 1 utilizing 3.1 parts by Weight of amorphous boron and 96.9 parts by weight of PbO, both to the same specifications given in Example 1. The mixture burns with explosive violence while releasing an average of 194 calories per gram. The grained material has an average bulk density of 1.9 g./cc.

out gas expansion chambers.

Example 17 A stoichiometric mixture of 4.05 parts, by weight of 9\0-97% purity boron and 95.95 parts, by weight, of Pb304 of %-plus purity was prepared by the procedure of Example 1. This mixture also burns with explosive violence. It releases an average of 294 calories per gram. The bulk density of the grained material averages 1.9 g./cc.

Mixtures such as in Examples 16 and 17 must be avoided in the construction of delay caps designed with- A violent or extremely -hot mixture can rupture the cap shell and causethe delay cap to fail to detonate by (a) failure to initiate the delay train, (b) snufng out the llame in the delay train, and/ or (c) complete separation of the ignition sectionV from the explosive section of lthe delay cap. Certain violent ignition systems can also cause malfunctioning of a delay cap by changing the pressure conditions within the atmospheric gases entrapped within the particles of the ignition mixture. -In a delay system, this pressure head factor contributes to the burning rate of the column of delay powder; hence, in these caps the pressure head must be maintained otherwise the desired time interval may not be attained. Proper time intervals between explosions in multiple hole blasting operations can be critical to the obtainment of the required results.

Advantageous properties of the ignition mixtures of the invention will be evident from the data in Table I which shows the results of comparative tests, namely, (a) Heat of reaction, (b) Ignition temperature, and (c) Voltage breakdown, conducted on typical compositions of the invention and the stoichiometric mixtures of Examples 16 and 17. Three other types of ignition charges conventionally used in blasting caps, namely diazodinitrophenol, a stoichiometric mixture of selenium and lead powder, and a mercury fulminate match-head type mixture were also tested. All compositions `and the DDNP were in free-owing form suitable for' use in electric blasting caps. The calorimeter tests were made in a No. 1411 Parr bomb filled with argon. gnition points were determined by dropping a sample of the test material onto aluminum foil oating on 'molten lead heated to the minimum temperature which would produce instantaneous ignition. Voltage breakdown tests were conducted in electric blasting cap assemblies of this invention (shown in FIGURES 1, 2, and 3 of the accompanying drawings) with the voltage applied to the shunted leg wires and the cap shell. These cap assemblies are described in detail further on in the specication.

From the data in the table, it will be apparent that the three preferred compositions, i.e., Examples 1, 5, and 11 and twelve usable, similar compositions, i.e., Examples 2-4, 6-10 and 12-l5, which have been made by the procedures of Example l have high levels of voltage breakdown when used in caps of the designs of the invention adequate to resist stray currents and static charges normally encountered during the use of caps in the eld. The compositions of the invention also have much higher ignition points than DDNP (Example 18), 27.6% Se/72.4% Pb (Example 19), or a gas-forming match composition (Example 20), hence, are less hazardous in this respect. The match composition releases gas when it is tired, hence, requires a gas expansion chamber. Since the Se/Pb mixture is conductive, a cap design, not now apparent, would bev required before stray current and static resistance could be attained with this mixture. The DDNP detonates when ignited in confinement, hence, its use is usually as an initiator for a high explosive charge. The stoichiometric mixtures, i.e., Examples 16 and 17, likewise react too violently Vto be used as ignition mixtures in blasting caps without provision of a gas expansion space or a conning means reactions.

TABLE I Ignition Mixture Data Cap Assembly Example No. Data Voltaze Composition, Percent Heat oi Instantaneous Breardwn,

by Weig t Reaction, Ignition Avg. A.C.

cal./g. Temp., C.

2% B/98% PbO.- 133 398 1, 400 1.5% B/98.5% PbO 1 N.R. 454 1,935 2 25% B/97.75% PbO.. N.D. 408 1, 970 2 5% B/97.5% PbO 158 412 2, 200 B/90% PbO 217 371 1, 470 8% B/92% PbO 202 380 1, 955 B/85% PbO- 190 420 2, 220 B/80% PbO-- 180 438 2, 030 [75% 171 N .D 1, 405 B/70% PbO 165 450 1, 450 25% B/75% P13304. 248 425 2, 060 8% B/92% P13304. N.D. 415 1, 840 15% B/85% PbaOi 268 434 1,800 20% B/S0% P13304 256 438 1, 880 30% B/70% P13304 N.D. 445 1,860 3.1% B/96.9% PbO 194 2 N.D. 2 N.D. 4.05% B/95.95% P13304... 294 2 N.D. 2 N.D. DiazodinitrophenoL N.D. 176 N.D. 27.6% Se/72.4% Pb.. 83 220 80 Match Composition. N.D. 204 1,820

1 No reaction in the argon atmosphere oi the calorimeter bomb.

interest.

As has been indicated previously, the compositions of the invention are particularly suitable for use in blast ing caps designed without an expansion chamber since they are non-violent and nongas-forming when ignited. An important aspect of the invention is a `blasting cap designed without an expansion chamber and for use in which the compositions of the invention are eminently suitable.

FIGURE 1 represents a cross-sectional view of a static and stray current resistant electric blasting cap in accordance with the invention.

Referring to FIGURE 1 which illustrates a cap of the Zero Delay type, item 1 is the shell containing the various components. A pressed charge 2 of PETN is at the base of the shell. Super-imposed on the base charge is a capsule 4 which contains most of the initiation charge 3 of DDNP so as to protect it from excessive pressure when it is compressed to form a sound base 5 for the ignition charge 6 which is the defined ignition charge of this invention. At 7 and 7a, a bridge- Wire is attached to the poles of lead wires 12. A section of dielectric tubing 9 is fitted to the lower half of bridge plug 8 and encompasses most of ignition charge 6, thus providing a longer potential arc path between the points 7 and 7a and shell 1 than the arc path provided between the lead wires 12 and shell 1 through the layer of porous dielectric 10. The contents of the cap are protected against ingress of water by the waterproof seal 11.

FIGURE 2 shows a cross-sectionalview of a delaytype electric blasting cap within the scope of the invention, the only difference from the cap illustrated in FIG- URE 1 being the presence of a delay column of fuse powder 13, interposed between initiation charge 5 and ignition charge 6.

FIGURE 3 is a cross-sectional view of a blasting cap similar to that shown in FIGURE 2, execept that delay column 13 is a core of fuse powder contained in a thickwalled tubular section 14.

It is seen that the present invention provides a novel class of ignition mixtures for electric blasting caps which are gasless and non-violent, thereby eliminating the need for a gas expansion chamber in the design of electric blasting caps. They are also non-conductive, thereby increasing the resistance of the cap in which they are used to premature ignition by stray currents and static discharges. Further, although differing quite widely in range `of composition, the mixtures are substantially uniform in their sensitivity to hot bridgewire and are, therefore, eminently adaptable for firing in series circuits. It will be appreciated that .besides blasting caps, the properties of the mixtures make them highly useful as ignition charges for electric squibs, electric detonators, electrically initiated heat motors, electrically initiated gas generators, explosive bolts, and the like.

Secondly, the invention provides a unique method for preparing the ignition mixtures whereby the use of a graining agent is eliminated.

Thirdly, the invention provides a new blasting cap assembly involving the use therein of the novel ignition mixtures which assembly greatly reduces the hazards of premature detonation caused by stray electric currents and static discharges normally associated with electric blasting caps and which in addition provides an important, economical advantage `by eliminating the need for `an expansion chamber in such devices.

While the present invention has been described herein in terms of specific examples and embodiments thereof, it is not intended that its scope be limited in any Way thereby, but only as indicated in the following claim.

We claim:

A method for preparing an electric blasting cap ignition mixture, consisting of (a) amorphous boron of 97% purity and (b) a member selected from the group consisting of PbO of greater than 99% purity and Pb3O4 `of not less than 90% purity, which comprises forming a wct intimate mixture of (a) and (b), drying the resulting mixture, crushing and forcing the dry mixture through a screen of 20-60 mesh size and electrostatically graining the screened mixture to substantially spherical form by tumbling in a container made of non-conductive material.

References Cited bythe Examiner 

